The present invention relates to high purity tin or tin alloy with reduced a dose of tin to be used in the manufacture of semiconductor manufacturing equipment or the like, and to a manufacturing method of such high purity tin.
Generally, tin is a material that is used in the manufacture of semiconductors, and in particular is a primary ingredient of solder material. When manufacturing semiconductors, soldering is used to bond a semiconductor chip and substrate, to bond or seal a Si chip such as an IC or LSI to or in a lead frame or a ceramics package, to form bumps during TAB (Tape Automated Bonding) or during the manufacture of flip chips, or on semiconductor wiring materials.
Since recent semiconductor devices are densified and of high capacity, there is considerable risk that a soft error may occur due to the influence of α rays from materials in the vicinity of the semiconductor chip. In light of the above, there are demands for high purification of the foregoing solder material and tin, as well as for the realization of materials with fewer α rays.
There are several disclosures relating to technology aiming to reduce α rays from tin. These technologies are described below.
Japanese Patent No. 3528532 (corresponding to Japanese Patent Laid-Open Publication No. H11-80852 A) describes a manufacturing method of low α ray tin including the steps of alloying tin and lead having an a dose of 10 cph/cm2 or less, and thereafter removing the lead contained in the tin by refining. The object of this technology is to reduce the a dose by diluting the 210Pb in the tin through the addition of high purity Pb. Nevertheless, in the foregoing case, such a complicated process is required as to additionally remove Pb after adding it to the tin. In addition, although the numerical value of the a dose is considerably low after the lapse of 3 years after refining the tin, this can also be comprehended as not being able to use the tin with the low a dose until 3 years has lapsed from the refining of the tin, and it cannot be said that this is an industrially efficient method.
Japanese Patent No. 3227851 (corresponding to Japanese Patent Laid-Open Publication No. H06-182580 A) describes that the radiation α particle count can be reduced to 0.5 cph/cm2 or less by adding 10 to 5000 ppm of a material selected among Na, Sr, K, Cr, Nb, Mn, V, Ta, Si, Zr and Ba to a Sn—Pb alloy solder. Nevertheless, even with the addition of the foregoing materials, the radiation α particle count could only be reduced to a level of 0.015 cph/cm2, and this has not achieved a promising level as a present-day material for semiconductor devices. Another problem lies in that elements such as alkali metal elements, transition metal elements and heavy metal elements which are undesirable to be mixed in the semiconductor are used as the additive materials. Therefore, there is no choice but to say that this is low level as a material to be used for assembling semiconductor devices.
Japanese Patent No. 2913908 (corresponding to Japanese Patent Laid-Open Publication No. H05-77087 A) describes reducing the count of radiation α particles emitted from solder ultra fine wires to 0.5 cph/cm2 or less, and using the same as the connection wiring of semiconductor devices. Nevertheless, this kind of count level of radiation particles is not at a level as a present-day material for semiconductor devices.
Japanese Patent No. 2754030 (corresponding to Japanese Patent Laid-Open Publication No. H02-228487 A) describes using highly refined sulfuric acid such as top-grade sulfuric acid and highly refined hydrochloric acid such as top-grade hydrochloric acid in order to obtain an electrolytic solution, and using high purity tin as the anode to perform electrolysis so as to obtain high purity tin having low lead concentration and a lead α ray count of 0.005 cph/cm2 or less. It goes without saying that a high purity material can be obtained by using a high purity raw material (reagent) without regard to cost, but the lowest α ray count of the deposited tin indicated in the Examples of Japanese Patent No. 2754030 (corresponding to Japanese Patent Laid-Open Publication No. H02-228487 A) is 0.002 cph/cm2, and an expected level cannot be achieved despite the high cost.
Japanese Patent Laid-Open Publication No. H11-343590 describes a manufacturing method for obtaining metallic tin of 5N or higher through electrowinning performed with the electrolytic solution that is obtained according to the procedures, which are precipitation of metastannic acid by adding nitric acid to a heated aqueous solution previously added with crude metallic tin, filtration, cleaning, and thereafter dissolution of such metastannic acid in hydrochloric acid or hydrofluoric acid. Although this technology vaguely describes its application in semiconductor devices, there is no specific reference to the limitation of U and Th as the radioactive elements or the radiation α particle count, and it could be said that the interest concerning the foregoing limitation is of a low level according to the disclosure of Japanese Patent Laid-Open Publication No. H11-343590.
Japanese Patent Laid-Open Publication No. H9-260427 describes technology of reducing the amount of Pb contained in Sn constituting the solder alloy, and using Bi, Sb, Ag or Zn as the alloy material. Nevertheless, even if the amount of Pb is reduced as much as possible in the foregoing case, Japanese Patent Laid-Open Publication No. H9-260427 does not particularly disclose any means for fundamentally overcoming the problem of the radiation α particle count caused by Pb that gets mixed in inevitably.
Japanese Patent Laid-Open Publication No. H1-283398 discloses tin having a grade of 99.99% or higher and a radiation α particle count of 0.03 cph/cm2 or less manufactured by performing electrolysis using a top-grade sulfuric acid reagent. Also in this case, it goes without saying that a high purity material can be obtained by using a high purity raw material (reagent) without regard to cost, but the lowest α ray count of the deposited tin indicated in the Examples of Japanese Patent Laid-Open Publication No. H1-283398 is 0.003 cph/cm2, and an acceptable level cannot be achieved despite the high cost.
Japanese Patent Laid-Open Publication No. S62-47955 describes lead as a brazing filler metal for use in semiconductor devices having a grade of 4N or higher, radioisotope of less than 50 ppm, and a radiation α particle count of 0.5 cph/cm2 or less. In addition, Japanese Patent Laid-Open Publication No. S62-1478 describes tin as a brazing filler metal for use in semiconductor devices having a grade of 99.95% or higher, radioisotope of less than 30 ppm, and a radiation α particle count of 0.2 cph/cm2 or less. Both Japanese Patent Laid-Open Publication No. S62-47955 and Japanese Patent Laid-Open Publication No. S62-1478 have a lenient allowable value concerning the radiation α particle count, and there is a problem in that the technology described in these Documents is not at a level as a present-day material for semiconductor devices.
Although Japanese Patent Laid-Open Publication No. 2001-82538 shows an example of Sn having a purity of 99.999% (5N), this Sn is to be used as a metal plug material for a seismic isolation structure, and there is no description concerning the limitation of U and Th as the radioactive elements or the radiation α particle count. This kind of material cannot be used as a material for assembling semiconductor devices.
Japanese Patent Laid-Open Publication No. H7-280998 discloses a method of eliminating technetium from nickel contaminated with a large quantity of technetium (Tc), uranium and thorium by using graphite or activated carbon powder. The reason for this is because, when attempting to eliminate technetium with electrolytic refining, technetium will follow nickel and codeposit on the cathode, and cannot be separated from such cathode. In other words, technetium as the radioactive material contained in the nickel cannot be eliminated with electrolytic refining. The technology of Japanese Patent Laid-Open Publication No. H7-280998 relates specifically to nickel contaminated with technetium, and cannot be applied to other substances. In addition, Japanese Patent Laid-Open Publication No. H7-280998 merely relates to low-level technology in terms of high purification of treating industrial wastes that are harmful to the human body, and is not at a level as a material for semiconductor devices.